1. Field of the Invention
The invention relates generally to a system and method for a strategy for the reduction of NOx emissions, and more particularly for the reduction of NOx emissions after fuel cut-off events in hybrid electric and conventional vehicles.
2. Background Art
During typical modes of operation, hybrid electric vehicles (HEVs) perform frequent engine shut off and restart operations. During engine shut downs and restarts, air is pumped into the exhaust system by the engine. A catalyst in the engine exhaust system adsorbs the oxygen from the air that is pumped into the exhaust system. Frequently, the oxygen is of sufficient volume to saturate the oxygen storage capacity of the catalyst. A three-way catalyst (TWC, i.e., a catalyst for simultaneous removal of CO, NOx and HC from the engine exhaust gas) that has saturated or nearly saturated oxygen storage capacity typically has diminished NOx conversion ability.
When combustion is resumed (i.e., when the engine is restarted), the NOx emissions from the engine are not efficiently reduced or eliminated by the catalytic converter until the oxygen storage level is reduced from the saturated level. In particular, a NOx spike can occur after an HEV engine restart.
Conventional methods for attempting to reduce the HEV restart NOx spike (i.e., limiting the number of times the engine shuts down) may not be totally effective. The NOx attributed to the HEV restart is a significant portion of the NOx standard for super/ultra low emissions vehicles (SULEV) vehicles.
Non-HEV vehicles (i.e., conventional vehicles) can also have a similar problem when shutting off fuel during high speed deceleration conditions (i.e., deceleration fuel cut-off). Restart NOx emissions are a problem during deceleration fuel cut-off of non-HEV vehicles as described above for engine restarts of HEVs. In both cases, as the engine spins to a low speed or stop a large amount of air can be pumped into the exhaust system (and catalyst). The large amount of air that can be pumped into the exhaust system can also cause undesirable catalyst cooling.
One example of a conventional approach for a method and apparatus for controlling an internal combustion engine capable of intermittent operations is disclosed in U.S. Pat. No. 6,742,327. The method and apparatus controls an internal combustion engine of a vehicle in which an exhaust purifying catalyst capable of storing oxygen is provided in an exhaust system of the engine. The internal combustion engine is adapted to be temporarily stopped when a predetermined condition for stopping the engine is satisfied, and resumes its operation when the predetermined condition is eliminated. The internal combustion engine is operated so as to reduce an amount of oxygen stored in the exhaust purifying catalyst during a temporary stoppage of the engine, before fuel starts being burned for resuming the operation of the engine.
However, such conventional approaches generally fail to efficiently reduce or eliminate NOx emissions from the engine until the oxygen storage level of the catalytic converter is reduced from the saturated level. In particular, such conventional approaches generally fail to reduce or eliminate the NOx spike that can occur after an HEV engine restart.
Thus, there is a need and desire for a system and a method to efficiently and effectively provide for controlling exhaust emissions to efficiently reduce or eliminate NOx emissions from the engine after fuel cut-off events.